In practice, manual and automatic shifting transmissions are known, which possess an input shaft, an output shaft coaxial to the input shaft and a countershaft for their operation. In accordance with the number of gear stages, gear pairings are to be found on the transmission shafts where, respectively, a fixed gear attached on a transmission shaft meshes with at least one loose gear placed on another transmission shaft.
As a rule, where a gear stage procedure is concerned, one of the loose gears is bound to its transmission shaft by way of a shape-fit clutch arrangement, which enables it to assume the entire driving torque. In order to transfer large driving torques with simple, space saving means, shape-fit clutches are advantageously used, which are easily shifted. However, during the engaging procedure, i.e., the shifting operation, the pulling power of the drive motor is interrupted due to a separated shifting clutch and a start clutch.
In order to shift such transmissions in ways that can be described as simple, free of impacts, quick and quiet, it is necessary that the components to be shifted be brought to nearly the same speed of rotation before they engage with one another. For this purpose, synchronization apparatuses are provided which, during the traction interruption phase, delay or accelerate the drive side of the input power string between the shifting/starting clutch and the respective coupling apparatus down to or up to a certain speed of rotation. This certain speed of rotation is determined by the current traveling speed and the transmission ratio at the desired gear stage. If a driver upshifts from a lower gear into a higher gear then, because of this the synchronization apparatus, the drive side part of the transmission is slowed. Conversely, if the driver downshifts, an acceleration thereof ensues.
For these accelerations and braking operations, the conventional synchronization apparatuses employ friction clutches based on friction cones. These are not required to transfer the entire drive-torque load, but are designed to carry out only the synchronization function, which arises from the momentum of the rotating masses of the drive-side part of the transmission as well as from the friction related drag-torque. On this account, synchronization apparatuses can be made in small dimensions.
Normally, to each of these shape-fit coupling devices, one synchronization apparatus is assigned. It is also possible that a central synchronizing-center can take over the necessary synchronizing function for several or for all of the shape fit coupling devices.
In order to relieve the driver of a vehicle from mechanical shifting and clutch operations involved with the above shifting methods, in the case of automatized shifting transmissions, these shifting methods and coupling procedures are assumed by auxiliary servo-apparatuses, which are under the direction of a control and regulation system. To fulfill its above stated purpose, the mentioned control and regulation system determines the shifting procedure reflecting the driver's desire from sensor acquired vehicle data and, accordingly, controls (or regulates) thereupon the succession of shifts to be carried out in the transmission with the aid of stored control and regulation programming.
In the case of such automatic transmissions, the synchronization procedure is, for example, so capable of being controlled and regulated that by downshifting procedures the speed of rotation of the input drive shaft of the transmission or the speed of rotation of the corresponding countershaft is made greater by increasing the rotary speed of the motor. Conversely, during upshifting procedures, these drive-side shafts of the transmission are subjected to braking. For the execution of such braking procedures, centrally synchronized transmissions normally rely on a “transmission brake”, which is mechanically coupled to the countershaft. These transmission brakes are activated by electrical, hydraulic or pneumatic power media. Pneumatically activated transmission brakes are commonly found in commercial vehicles.
DE 196 52 916 A1 discloses an automatized shifting transmission with a hydraulically or pneumatically activated transmission brake, wherein the pneumatically activated brake can be applied to a countershaft. The control valves for the hydraulic pressure fluid to serve the brake are regulated by a microprocessor which is programmed for the desired method of shifting as well as for the other driving operations.
For example, where an upshifting is concerned, if a higher gear stage has been preselected and the countershaft, on this account, must be subjected to braking for synchronization purposes, then a set speed of rotation (synchronized speed of rotation) for the countershaft is computed by the microprocessor which utilizes the preselected gear ratio and the corresponding output speed of rotation of the transmission, which is picked up by a sensor. Upon achieving the above values, then the mechanical coupling of the targeted loose gear to this shaft can be carried out.
Because of the difficult regulation of air pressure which, as a rule, is the case when pneumatic servomechanisms are concerned, plus additional problems from changeable ambient conditions, the transmission brake capacity and particularly the rate of change of such pneumatic transmission brakes are subject to large variances. In order to attain the required window of speed of rotation, i.e., to attain the preset maximum difference between the actual speed of rotation of the transmission input drive shaft—that is, of the rotational speed of the countershaft driven by the input drive shaft—to the set speed of rotation for the desired shifting procedure, in accordance with the prior state of the technology. Additionally, even the rate of change of the transmission brake must be determined and be input to the microprocessor upon control of the transmission brake. To this end, the control valves are so regulated by the microprocessor that they achieve the set values and therewith the synchronized speed of rotation is determined for an engagement of the coupling apparatus to the selected loose gear.
Unfortunately, a synchronized speed of rotation is not a fixed value for one shifting procedure but, among other considerations, is dependent upon the inclination of the traveled road. The reason for this is that during a shifting procedure with an open starting and shifting clutch, as well as climbing on a rising road, a negative vehicle acceleration occurs which leads to a drop of the transmission output rotational speed or, on the other hand, upon a downward sloped stretch of road and with untouched wheel brakes, a positive vehicle acceleration can come about. These stated influences were not previously taken into consideration in the case of the control and regulation procedures for transmission braking according to the state of the technology. Consequently, the desired operational results were never fully consummated.
On this account, a proposal was made by the (not published) text of DE 103 05 254 A1 that, in addition, to the previously stated values for the control and regulation of the transmission brake, the rate of change of the speed of rotation of transmission output shaft should also be evaluated. By this method of procedure, the speed of rotation of the transmission output shaft with the ratio of the desired gear stage is related to the speed of rotation of the countershaft and, further, the ambient environment and/or the influence of the characteristics of the traveled road are being given more consideration than before. Especially during the shifting procedure, occurring rapid changes of the transmission output shaft speed of rotation can also be brought into the control and regulation of the transmission braking.
Moreover, the said DE 103 05 254 A1 also teaches that, first, from the rate of change of the transmission input speed of rotation, which is also that of the countershaft, and second, from the rate of change due to the influences, as mentioned above, on the changing transmission output shaft speed of rotation, a so-called composite rate of change may be computed, which contributes to the calculation of the disengagement point in regard to the activation time required for the transmission brake upon the attainment of the synchronization point. This disengagement point of time is, by comparison, even under varying transmission brake friction values and under output speed of rotation variance rates of change, capable of being way for the exact determination of a safe, rapid and jerk free shifting procedure.
Even though, by means of the above described procedure, the operational behavior of transmission braking is already clearly improved, there also exists additional optimizing possibilities.
Given this background, it is the purpose of the invention, to continue the improvement of the operational behavior of a transmission brake and therewith also to better the shifting method of an automatic transmission, especially with an upshifting procedure so that an optimal disengagement point can be better determined than formerly.